1. Field of the Invention
The present invention relates to a drive control apparatus for a hybrid vehicle whose drive shaft is rotated by an engine and an electric motor, and more particularly to a drive control apparatus for a hybrid vehicle which has two motors for applying drive force (torque) to front and rear wheels of the hybrid vehicle.
2. Description of the Related Art
In recent years, hybrid vehicles having drive wheels rotated by an internal combustion engine and an electric motor have been widely developed. The hybrid vehicle operates in various modes including a so-called EV (Electric Vehicle) mode, in which the engine is disabled and the hybrid vehicle is run only by the motor, for reducing fuel consumption by the engine.
When the hybrid vehicle needs a greater torque, or the electric power remaining in a battery on the hybrid vehicle is low, the engine is started. At this time, the battery is required to supply electric power large enough to start the engine. Therefore, it is necessary to monitor the electric power that remains in the battery, and to control the drive power of the motor, taking into account the electric power that is needed to start the engine.
There has been proposed a system which is a redundant structure having a propulsive motor, and a conventional dedicated starter motor for starting an engine, which works in complement with the propulsive motor (see, for example, Japanese laid-open patent publication No. 10-136508).
When a starter motor starts an engine, the starter motor needs to consume a large amount of electric power, causing a voltage drop across a battery that is connected to the starter motor. Therefore, a system has been proposed to inhibit the starter motor from operating while the hybrid vehicle is running, so that the operation of a controller for controlling the propulsion of the hybrid vehicle will not adversely be affected when the battery voltage drops (see, for example, Japanese patent No. 2,973,797).
On the hybrid vehicle, the motor may be operated in a regenerative mode as an electric generator for charging the battery. When the motor generates electric power in the regenerative mode, it is desirable to distribute as much drive power as possible from the drive wheels to the motor, and any resistance to intake air and exhaust emission of the engine should be small. In order to reduce the resistance to intake air and exhaust emission of the engine, there has been proposed a technique for disabling engine cylinders by inactivating intake and exhaust valves of the engine cylinders (see, for example, Japanese laid-open patent publication No. 2002-201972). According to the proposed technique, a sufficient amount of regenerated electric power can be obtained, an optimum exhaust gas control process is not adversely affected, and fuel consumption is improved.
In the system disclosed in Japanese laid-open patent publication No. 10-136508, the propulsive motor is supplied with electric power from a high-voltage battery, and the starter motor is supplied with electric power from a low-voltage battery. Since the low-voltage battery also supplies electric power to a propulsion controller other than the starter motor, when the engine is started, the voltage across the low-voltage battery drops, possibly affecting the operation of the propulsion controller. Further, with the system disclosed in Japanese patent No. 2,973,797, the engine cannot be started while the hybrid vehicle is running.
If the high-voltage battery is used to start the engine, then the voltage across the low-voltage battery does not drop when the engine is started. In the EV mode, it is necessary to take into account the possibility of starting the engine, and the electric power that can be supplied to the propulsive motor is equal to the difference between the total electric power that can be supplied from the high-voltage battery and the electric power that is required to start the engine. Therefore, a speed range in which the hybrid vehicle can travel in the EV mode, i.e., a rotational speed range of the propulsive motor, is relatively narrow, making it difficult for the hybrid vehicle to give its expected performance sufficiently.